> Can you recommend a chair

No.

I'm going against the grain of most replies here. It isn't a specific chair you need (perfectly good, bog standard office chairs are fully adjustable and ten a penny) but to instead take regular stretch breaks and don't slouch.

If I was a Silicon Valley geek, I'd recommend regular 10 minute yoga breaks, but since I'm British I find frequent walks around the garden with a cup of tea do the job just as well :-)